Path: news.mathworks.com!not-for-mail From: "Steven_Lord" <slord@mathworks.com> Newsgroups: comp.soft-sys.matlab Subject: Re: suppose I have an equation x^2+x+y^2+3*y=5*y^2+3*y+3*x*y+5 and I want to find the set of (x,y) that satisfy the equation. Date: Thu, 12 Aug 2010 10:46:14 -0400 Organization: MathWorks Lines: 43 Message-ID: <i411fm$4e0$1@fred.mathworks.com> References: <i40rce$qur$1@fred.mathworks.com> NNTP-Posting-Host: ah-slord0w.dhcp.mathworks.com Mime-Version: 1.0 Content-Type: text/plain; format=flowed; charset="UTF-8"; reply-type=response Content-Transfer-Encoding: 7bit X-Trace: fred.mathworks.com 1281624374 4544 172.31.44.22 (12 Aug 2010 14:46:14 GMT) X-Complaints-To: news@mathworks.com NNTP-Posting-Date: Thu, 12 Aug 2010 14:46:14 +0000 (UTC) In-Reply-To: <i40rce$qur$1@fred.mathworks.com> X-Priority: 3 X-MSMail-Priority: Normal Importance: Normal X-Newsreader: Microsoft Windows Live Mail 14.0.8089.726 X-MimeOLE: Produced By Microsoft MimeOLE V14.0.8089.726 Xref: news.mathworks.com comp.soft-sys.matlab:661427 "Kittithad Wangveerathananon" <kittithad@startfromyou.com> wrote in message news:i40rce$qur$1@fred.mathworks.com... > Question > How can i do that? > I've been trying, solve, explot and subs. > I can get the graph, but i cannot get the (x,y) pairs that I want. What, specifically, do you want to obtain? There are an infinite number of (x, y) points that satisfy this equation. One such pair is approximately (3, -2.8615). > When I use function solve to solve the equation in term of x or y and then > substitute the numeric value for one of them to find the other, I got the > problem about complex numer. This might be because that numeric value that > I substitute is not the number that satisfy the equation, so the program > give me the result in complex number. > > Background. > I need to use the pairs to find the optimal value of my objective > function. > I tried to use the fmincon, but it gave me the optimal value that is not > reliable, so i try to do it manually. > Note that my equality constraint is in the form of the equation in the > subject. Don't do that. Please put your ENTIRE question in the body of your message, for easy quoting. > I also have a couple of inequality constraints. If you were just looking to identify the points that satisfy your equation graphically, I would use CONTOUR. Write your equation as z = f(x, y) and look at the contour for z = 0. -- Steve Lord slord@mathworks.com comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ To contact Technical Support use the Contact Us link on http://www.mathworks.com